The suggestions in the Graduale or Liber are generally good, but they are merely suggestions. Apart from that, sing them yourself and listen to them - see if they fit the mood of the season/feast. Generally, the more melismatic Masses should be for days of higher solemnity.

Also important - if you're just beginning with a Gregorian Mass, don't try to get your schola (and/or congregation) to learn too many settings of the Ordinary at once. If I were just starting, I would only do at most three or four different Ordinaries during the course of the first year and slowly introduce more (if you wanted) from there on out. I actually think you can get by with four or five in a year (one for Advent and/or Lent, Christmas Time, Easter Time, Feasts/Solemnities, and Ordinary Time).